Method and device for supplying electrical power

ABSTRACT

Method and device for supplying electrical power to a wafer that is at least partially submerged in a liquid. The device comprises: —a container filled with the liquid; —a transportation device comprising a wafer carrier device for transporting the wafer at least partially submerged through the liquid; —a power supply device for supplying electrical power to the wafer.

FIELD OF INVENTION

The present invention is related to a method and device for supplying electrical power to a wafer. More specifically, the invention is related to a method and device for supplying electrical power to a wafer at least partially submerged in a liquid.

BACKGROUND OF THE INVENTION

A device and method for exposing wafers to a liquid are described in GB patent application 0709619.1 filed on 18, May, 2007. This application is hereby incorporated by reference.

In some processes there is a need to supplying electrical power to the wafer while the wafer is at least partially submerged in the liquid. One example of such a process is electroplating, where for example Ni, Cu, Sn and/or Ag is applied to the wafer. A challenge in this process is to provide electrical contact to the wafer without applying strong mechanical forces to the wafer, which may cause breakage of the wafer. Another disadvantage in such processes is that the metal or other materials are deposited on other parts of the production equipment, and this has to be removed periodically, which increases the non-productive time for the equipment.

A solar cell wafer has normally a thickness between 160-180 μm, where there is a continuous development towards producing thinner wafers. These wafers are fragile, and must be handled very carefully to avoid breakage. At the same time there it is a desire to increase the throughput of cells in an industrial process.

The object of the invention is to provide a method and device for supplying electrical power to a wafer, where these disadvantages are avoided. Moreover, it is an object of the invention to improve the efficiency of this working operation, and hence the overall production capacity, while at the same time avoid breakage of wafers.

SUMMARY OF THE INVENTION

The present invention is defined in the enclosed independent claims. Further embodiments are described in the dependent claims.

DETAILED DESCRIPTION

In the following, embodiments of the invention will be described with reference to the enclosed drawings, where:

FIG. 1 illustrates an overview of an embodiment of the invention;

FIG. 2 illustrates an enlarged view of the left side of FIG. 1;

FIG. 3 shows a perspective view of the holding device in FIG. 2;

FIG. 4 shows a side view of the holding device in FIG. 2;

FIG. 5 illustrates another embodiment of the invention;

FIGS. 6 a and 6 b illustrate two cross sectional views of the holding device.

In FIG. 1 it is shown a device comprising a transportation device 10 over a liquid container 2 containing a liquid. The transportation device 10 comprises wafer carrier devices 12 mounted between two transportation bands 14 a and 14 b.

The transportation device is further comprising a drive system (not shown) for moving the transportation bands and consequently also the wafer carrier devices 12 over the liquid container 2 as illustrated by arrows A.

Wafers 3 are picked up or are in other ways fastened to the wafer carrier device 12 near a first end of the container 2 (on the left in FIG. 1) and is being held by the transportation device 10 during transport to the second side of the container 2 (on the right in FIG. 1), where the wafers are released and for example transported to a next subprocess facility. During the transportation, the wafers 3 are completely or partially submerged into the liquid.

In FIG. 1 it is shown that three wafers 3 are transported in parallel. The transportation device 10 may in this embodiment comprise separation means 16 to keep the parallel wafers 3 separated from each other. It should be noted that not all separations means 16 are shown in the drawings, some of them are omitted for clarity.

Of course, it would be possible to transport only one wafer 3, or it would be possible to transport more than three wafers 3 in parallel.

The wafer carrier devices 12 comprises holding devices 18 with a cross section substantially formed as a dove tail joint (as shown in detail in FIGS. 3 and 4), i.e. the holding device 18 comprises wedge-shaped grooves 19 in the longitudinal direction of both sides of the holding device. The end of the wafer 3 is received in the wedge-shaped grove 19. As shown in FIG. 2, a wafer 3 is held between two holding devices 18 of respective wafer carrier devices 12.

There are several ways of arranging and designing such a transportation device 10 and wafer carrier devices 12, and many such arrangements are shown in GB patent application 0709619.1 filed on 18, May, 2007 which is hereby incorporated by reference.

The holding device 18 further comprises electrical contacts 20 localized for example in or near the grooves 19 to provide electrical contact with the wafer while the wafer is being held by the holding device 18. The electrical contacts 20 are placed at intervals along the grooves 19, and can be applied either on the top side of the wafer, bottom side or both. It is also in principle possible to establish a different voltage on the two surfaces of the two wafers if desired from a process point of view, for example for plating. Further, the electrical contacts are connected to a power supply 32 by means of electrical wires 30, as illustrated in FIG. 2 (indicated by dashed lines).

In FIG. 2 it is shown that the electrical contacts 20 are connected to the negative terminal of the power supply 32, while the positive terminal is connected by means of an electrical wire 34 to an electrical conductor or anode 36 submerged in the liquid.

It should be noted that since the transportation device rotates, means should be provided to avoid winding of the electrical wires 30 between the power supply and the holding devices. For example could bus bars connected to the negative terminal of the power supply be provided over the liquid. In this case, the holding devices 18 would be adapted to get in contact with the bus bars over the liquid for the desired contacting time, and consequently also provide electrical contact between the wire 30 and the electrical contacts 20.

It is important to achieve good electrical contact between the wafers and electrical contacts of the holding device. One possibility is to use spring loaded electrical contacts or electrical contacts comprising soft brushes inside or around the grooves in the holding device. Another possibility is to direct the liquid flows over and below the wafers so that the total flow pattern creates an automatic upward or downward pressure on the wafer. In the above-mentioned GB patent application 0709619 it is described how channels or carve-outs in the holding devices may be used to distribute liquid flows between the two sides of the wafer. Such channels are denoted with reference number 50 in FIG. 3 and FIGS. 6 a and 6 b. “Upward” channels (FIG. 6 a) is leading the liquid from the bottom part of the bath to the top side of the next wafer will create a higher liquid level on the top side of the wafer than in the open areas between the wafers, and consequently create a downward pressure on the wafer. “Downward” oriented channels (FIG. 6 b) will increase the pressure on the bottom side of the wafers and create an up-lift on all wafers. The liquid can be flowing through the channels by means of the movement of the holding device through the liquid bath, or by means of pumping means.

Another option for providing a downward acting force on the wafer could for instance be to provide a system where the liquid in the vessel is pumped from below the wafers and transferred via piping with outlets or nozzles above the wafers. Such an arrangement would create a small difference in pressure between the liquid on top of the wafers and the liquid below the wafers. The pressure difference will result in a net force on the wafers acting downwards. The required force for enabling a good mechanical and electrical contact to the wafers could then be adjusted by the velocity of the liquid flow.

Surface tensions may furthermore cause partially submerged wafers with dry top side to have an uplift relative to the holding devices while the same wafers with a wet top side will resist being lifted. Both mechanisms can be used to ensure a good electrical contact.

Moreover, it would be possible to provide several bus bars over the liquid, where the respective bus bars are having a different potential or voltage level in relation to the anode 36.

The electroplating process will now be described with reference to FIG. 2. Here, the liquid container is omitted for clarity, but it should be mentioned that the rightmost wafer 3 a is at least partially submerged in the liquid. The vessel contains a liquid in which metal ions, M^(n+), are present. The aim with the invention is to use prior art referred to as electroplating, for depositing metals thorough an aqueous solution onto the surface of silicon wafers for producing contacts for collection of current generated in the wafers when illuminated (solar cells). The metal ions in the solution is reduced to solid metal, M(s) at the surface of the wafer by applying the negative pole (cathode) of a power supply to the wafer. The positive pole (anode) of the power supply is submerged into the liquid, which in electroplating commonly is referred to as the plating bath or the electrolyte. The anode typically consists of the same metal that is dissolved in the electrolyte. When applying a current through the electrolyte two main reactions occur; one at the anode and one at the cathode:

Cathode Reaction: M^(n+) +ne ⁻→M(s)  (1)

Anode reaction: M(s)→M^(n+) +ne ⁻  (2)

Hence, the anode goes into solution into the electrolyte via an oxidation reaction while the same time metal ions are being reduced to solid metal at the cathode.

In the first embodiment, the holding devices are made of an insulating material e.g. PP, PVDF or PTFE or other suitable material. Inside the holding devices there are smaller pins of a conductive material e.g. stainless steel or titanium. Those pins are designed in such a way that they act as support to the wafer while at the same time providing an electrical contact to the side of the wafer facing downside towards the bottom of the vessel. The contacting devices could have the shape of needles, balls, rods, discs or any other geometrical dimension that enables a good contact interface and maintaining low mechanical impact on the wafer.

The contacting devices are via a conductive material such as copper connected to the negative terminal of a power supply (cathode). The positive terminal (anode) of the power supply is connected to a piece of metal that is submerged in the liquid vessel that contains the electrolyte. When switching on the power supply, there will be a circuit created where the wafer is the cathode (reaction 1).

In this way some relevant metals for solar cell manufacturing, including e.g. Ag, Cu, Sn and Ni, could be deposited in a cost efficient way from aqueous solutions commercially available. The wafer handling system can be designed in such a way that it would not be necessary to change to another transport system (no new holding device) for each metal that you want to deposit.

It should be mentioned that during the process, metal will deposit on the electrical contacts 20. The metal could be removed physically (for example by means of a brush) or chemically (for example by means of an acid), as previously known.

However, the present invention provides a novel way of improving this. The metal could be removed electrically for example by reversing the current direction or connecting the holding device to a bus bar with a suitable (opposite) potential, for example after the wafer has been removed from the holding device. The electrical contacts 20 would in this case be submerged into a suitable liquid (the same or a second liquid bath). The removal of the metal can then be integrated as part of the process, and consequently the non-productive time for the production equipment will be reduced.

Another embodiment of the invention would be to have a brush-like contacting arrangement 40 (shown in FIG. 5) comprising current conducting wires 42 electrically connected to the power supply and in contact with the upper surface of the wafer during transportation. In this way the different areas of the surface of the wafer will be closer to the current conductor. The current conducting wires can be flexible to avoid damage on the wafer. The method is primarily relevant when or if the electrical conductivity of the wafer surface between the two holding devices on the edges is not sufficiently good for edge contacting alone.

In another embodiment of the invention, lights are providing situated either submerged in the liquid or above the wafers for enabling so called light induced plating.

Common Features

The abovementioned detailed description is especially provided to illustrate and to describe embodiments of the invention. However, the description is by no means limiting the invention to the specific embodiments.

It would of course be possible to change the polarities of the power supply device 34, or to use a alternating current instead of a direct current power source.

In fact, in one application of the invention, it would be possible to use the above described configuration in a first liquid container, and thereafter use the reverse polarity in a second liquid container to remove metal remnants etc. 

1-27. (canceled)
 28. Device for supplying electrical power to a wafer that is at least partially submerged in a liquid, comprising: a liquid container filled with the liquid; a transportation device comprising at least two wafer carrier devices for transporting the wafer at least partially submerged through the liquid from a first side of the container to a second side of the container; a power supply device for supplying electrical power to the wafer; where the wafer carrier devices comprises holding devices comprising wedge-shaped grooves for holding edges of a wafer between two holding devices of the respective wafer carrier devices; and where the wedge-shaped grooves comprises electrical contacts for contacting the wafer, where the electrical contacts are in electrical connection with the power supply device.
 29. Device according to claim 28, where the electrical contacts are connected to a first terminal of the power supply device and where a second terminal of the power supply device is connected to the liquid.
 30. Device according to claim 29, where the electrical contacts are spring-loaded to improve the electrical contact with the wafer.
 31. Device according to claim 28, where the holding devices comprises channels over or under the wedge-shaped grooves, for providing a force to the wafer by means of liquid flowing through the channels.
 32. Device according to claim 28, where the device further comprises a system comprising inlets, pumping means, pipings and nozzles, for pumping liquid from the inlets located below the wafers out through to the nozzles located above the wafers, via the pumping means and the pipings.
 33. Device according to claim 28, wherein the device comprises bus bars for providing contact between the wafer and the power supply.
 34. Device according to claim 33, wherein the bus bars can have different voltage potentials for different wafer positions.
 35. Device according to claim 28, wherein the device comprises a second liquid container for providing a reverse-plating process to remove deposited metal.
 36. Method for supplying electrical power to a wafer that is at least partially submerged in a liquid, comprising the following steps: a) holding the wafer between two holding devices by receiving the ends of the wafer in a wedge-shaped opening of the respective holding devices; b) transporting the wafer at least partially submerged through the liquid by means of a transportation device from a first side of the container to a second side of the container; c) supplying an electrical power to the wafer by means of electrical contacts localized in or near the wedge-shaped grooves.
 37. Method according to claim 36, further comprising the step of providing that the electrical contacts are connected to a first terminal of the power supply and a second terminal of the power supply device is connected to the liquid.
 38. Method according to claim 36, further comprising the step of providing the electrical contacts as spring-loaded electrical contacts to improve the electrical contact with the wafer.
 39. Method according to claim 36, further comprising the step of providing the holding devices with channels over or under the wedge-shaped grooves, for providing a force to the wafer by means of liquid flowing through the channels.
 40. Method according to claim 36, further comprising the step of providing a spraying arrangement for pumping liquid through nozzles directed towards the surface of the wafer.
 41. Method according to claim 36, further comprising the step of providing bus bars for providing contact between the wafer and the power supply.
 42. Method according to claim 41, further comprising the step of providing the bus bars with different voltage potentials for different wafer positions.
 43. Method according to claim 36, further comprising the step of providing a second liquid container for providing a reverse-plating process to remove deposited metal.
 44. Method according to claim 36, further comprising providing lights situated either submerged in the liquid or above the wafers for enabling so called light induced plating. 